1. Field of the Invention
The present invention relates generally to a press die and a molding process of a disk with shaft shaped portion for press molding a glass substrate of a disk formed from a disk member having a shaft shaped portion at a center, such as an optical disk, a magnetic disk and so forth.
2. Description of the Related Art
In the recent years, demand for higher performance of a disk drive unit as external memory device, such as down-sizing, higher capacity and so on, has been growing significantly with the spread of computers. Association with this, lowering of floating height of a magnetic head has been aggressively studied to require quite high precision for flatness of the magnetic disk.
Conventionally, as a material of substrate for a magnetic disk, aluminum alloy is mainly employed. However, in case of aluminum alloy substrate, a polished surface may cause plastic deformation even when high precision polishing is provided by high precision abrasive forming and polishing device due to low hardness. Therefore, it is difficulty to obtain the flat surface on the aluminum alloy disk with satisfactorily high precision. Also, even when a nickel-phosphorous plating layer having higher hardness is formed on the surface of the substrate, it is still not possible to satisfy the demand for high precision.
On the other hand, movement for down-sizing requires thinning of a disk thickness. The aluminum alloy substrate having low strength is difficult to satisfy the demand in this viewpoint.
Employment of giant magnetoresistive head having high sensitivity in the recent years, requires reduction of noise of the magnetic disk. For this reason, the substrate is required to adapt for noise reduction process by heat treatment after formation of magnetic film. However, even in this viewpoint, it has been difficult for aluminum alloy to satisfy the demand.
For solving the foregoing problems, as the substrate for the magnetic disk, new materials, such as glasses, ceramics, carbon and so on have been proposed. Among these new materials, glass substrates have been widely studied and have already been put into practice. The glass substrates has high strength, high heat resistance, high surface hardness sufficient for satisfying demand for high precision flatness by precise polishing process.
Conventionally, the glass substrate for the magnetic disk has been produced by polishing method to effect precise polishing for each individual glass substrate in one by one basis for obtaining flat surface after cutting into a predetermined size. However, high precision is required in the polishing process and large number of process steps are required.
Therefore, as new fabrication process of the glass substrate for the magnetic disk, Isothermal press molding process for perform press molding with equal temperature of die and glass material has been considered. Various studies have been made for the isothermal press molding process in the field of optical glass element fabrication. As a result, the isothermal press molding processes have already been put into practice as fabrication processes capable of fabrication of the glass substrates with high quality and high productivity.
The press die to be used for press molding has to be a special die which does not cause deterioration even by repeated molding of glass at high temperature and high pressure. Therefore, various studies have been made for the press dies.
As a matrix of press die for press molding, hard metal (tungsten carbide), cermet, zirconium, silicon carbide and other ceramics may be used. On the other hand, the die coated with a protective film having high mold releasing ability, oxidation resistance, reaction resistance has been developed for protection of the matrix and prevention of adhesion of glass upon mold release.
For example, Japanese Patent Application Laid-open No. 2-137914 (1990) proposes a die for molding a disk provided with a noble metal alloy thin film on a surface of hard metal. This process has been actively studied as fabrication process of the glass substrate for the magnetic disk corresponding to movement of recent high density recording.
On the other hand, recent demand for large recording capacity for the disk of a disk drive unit requires high precision of precision in rotation of the disk.
However, in the disk drive unit mounting annular disks on a rotary base rotatingly driven by a disk drive motor in the conventional construction, surface run-out of respective flange portion and disk of the rotary base and axis run-out relative to rotation center appear in collective manner. It is quite difficult to restrict dynamic surface run-out and axis run-out on the disk in rotation. It would require substantially high cost for producing the disk drive unit for restricting surface run-out and axis run-out.
On the other hand, in the recent years, demand for mobile application, such as personal computer, is growing to promote further reduction of size and thickness of the disk drive unit. However, the conventional construction cannot satisfy the demand set forth above.
In this circumstance, the inventors have made extensive study for realizing reduction of thickness of the disk drive, realizing high precision control of surface run-out and axis run-out, realizing high recording density by integrally forming the disk and the rotary shaft of the disk drive motor, and thus providing a shaft integrated type glass disk substrate through a glass molding process.
For example, a structure of die for press molding of a glass substrate of the disk constructed from a disk member having a shaft shaped portion at the center portion of the optical disk, magnetic disk and so on has been illustrated in FIGS. 22 and 23, for example. Also, the press molding process is illustrated in FIGS. 24 to 26.
In the first conventional example, as shown in FIGS. 22 and 25, the die for performing press molding is constructed with an upper die 114 for press molding an upper surface of a disk member 125, and a lower die 116 preformed with a shaft forming hole portion 116a of the shape corresponding to a shaft shaped portion 124 for press molding the shape of the shaft shaped portion 124. The die is used in combination with a height restriction plate 115 for restricting thickness of the disk member 125 when the die is pressed. The shaft forming hole portion 116a is not formed as a through hole but is provided as recessed hole.
In the practical press molding operation, as shown in FIGS. 23 to 26, a nitric material 122, such as glass is placed between the upper die 114 and the lower die 116. Then, positioning of the upper die 114 and the lower die 116 is adjusted by means of the height restriction plate 115. The height restriction plate 115 also serves as guide for the upper die 114 and the lower die 116. The upper die 114 and the lower die 116 are sandwiched between stages 117 and 118 which are controlled in temperature by heaters 119 and 120. The stages 117 and 118 are heated by the heaters 119 and 120 for heating the upper die 114 and the lower die 116 to heat the nitric material 122 up to a temperature (near glass softening temperature) for performing glass molding.
Next, deformation is caused in the nitric material 122 due to application of pressure by a press 121 to form a molded product 123 formed from a disk member 125 illustrated in FIGS. 24 to 26.
However, the tip end of the shaft shaped portion 124 of the molded product 123 formed by the die structure set forth above cannot be forms into the shape accurately corresponding to the lower die 116 due to presence of residual air which cannot be discharged through the shaft forming hole portion 116a of the lower die 116 upon molding. This influences for the precision of shape of the shaft portion. Furthermore, in the worst case, the edge portion can be formed in reversed arc-shape. On the other hand, in design, it is possible to eliminate influence of the residual air. However, it becomes necessary to perform post process, such as cutting of the tip end portion 124a of the shaft shaped portion 124.
On the other hand, even in the disk member 125, the residual air for the shaft portion during molding step also affects the disk portion and makes flatness and circularity of the disk member 125 insufficient for the demanded precision.
For this reason, it becomes necessary to mold a disk having greater diameter than the desired size and to perform post process such as treatment for the outer periphery portion of the molded product 123 or treatment for the outer shape of the disk member 125.